The proposed experiments will examine the cellular mechanisms of experience-dependent synaptic plasticity, changes in synaptic function which are widely posited to be necessary for learning and memory. These experiments will be done in the rodent whisker-barrel system using the unusual approach of studying the same cell populations in vivo and in vitro. Following whisker hemi-deprivation (2-3 rows out of 5 on one side of the face), cortical-cell response characteristics will be studied in the barrel fields, the cortical representation of whiskers. These studies are designed to answer critical questions regarding specific synaptic dynamics and competition across a visible border between cortical areas that represent spared and deprived whiskers. First, in vivo extra- and intracellular recordings of cells in each barrel column will be performed to describe the synaptic activity of cells in deprived and spared cortical regions. Second, in a novel slice preparation (Finnerty et al., 1999), whole-cell recordings will be made from pairs of identified neurons that are synaptically coupled across adjacent barrel columns to discern whether changes in synaptic strength are due to alterations in quantal size, number of transmitter release sites, or probability of transmitter release. These dynamics will be examined at both excitatory and inhibitory synapses. In all experiments, comparisons of cells crossing the border between deprived and nondeprived cortex and those away from the border will be made. In the paired cell recordings, responses will be examined in both directions across this border. The results will give a uniquely detailed view of the synaptic changes underlying sensory cortical plasticity.